An image pickup device for use in a digital single-lens reflex camera, a video camera, or the like has been downsized in terms of pixel size, and therefore an optical signal output from each one of pixels has become small. Thus, noise reduction has been increasingly demanded so as not to lower the S/N ratio.
FIG. 13 schematically shows in block diagram the construction of an image pickup device (CMOS). As shown in FIG. 13, the image pickup device includes a pixel region 601 formed by a plurality of pixels 602 arranged in a two-dimensional matrix. In the pixel region 601, incoming optical signals from an object are converted into electrical signals (image pickup signals).
FIG. 14 shows the circuit construction of each one of the pixels 602. Referring to FIG. 14, a photodiode 701 of the pixel 602 receives a part of an optical image formed thereon by a photographing lens, not shown, and generates and stores an electric charge. The pixel 602 includes a transfer switch 702, a reset switch 703, an amplifier 704, and a selection switch 705 each of which is formed by an MOS transistor.
The transfer switch 702 has a gate thereof input with a corresponding one of drive signals tx1 to tx4 output from a vertical scanning circuit 603 of the image pickup device. The electric charge stored in the photodiode 701 is input via the transfer switch 702 to a gate of the amplifier 704 that converts the electric charge into a voltage which is output from a source follower. The gate of the amplifier 704 is reset by turning on (closing) the reset switch 703. The selection switch 705, when turned on, outputs a pixel signal to a corresponding one of vertical output lines H1 to H4 in FIG. 13.
Referring to FIG. 13 again, the vertical scanning circuit 603 outputs drive signals res1 to res4, tx1 to tx4, and sel1 to sel4 to the pixel region 601.
In accordance with the drive signals from the vertical scanning circuit 603, signal components S and noise components N are output from the pixels 602 of the pixel region 601 to the vertical output lines H1 to H4.
The vertical output lines H1 to H4 are connected to a readout circuit 604 of the image pickup device. The signal components S and the noise components N read out from the pixels 602 are temporarily held in the readout circuit 604.
The noise component is a pixel output, which is generated immediately after the reset switch 703 is turned on in accordance with the drive signal and which is then held in an N signal holding section of the readout circuit 604 via the amplifier 704, the selection switch 705, and a corresponding one of the vertical output lines H1 to H4 in a state that the transfer switch 702 is turned off. The noise component includes reset noise generated when the reset switch 703 is turned on, a variation between the pixels in gate-to-source voltage of the amplifiers 704, and so on.
The signal component is an electric charge, which is transferred from the photodiode 701 when the transfer switch 702 is turned on in accordance with the drive signal and held in an S signal holding section of the readout circuit 604. A noise component generated when the transfer switch 702 is turned on is added to the signal component.
Subsequently, a horizontal scanning circuit 605 of the image pickup device operates to output the signal component S and the noise component N to a differential amplifier 608 via respective ones of MOS transistors 606, 607.
A difference between the signal component S to which a noise component is added and the noise component N is amplified by and output from the differential amplifier 608, whereby a pixel output from which the noise component is removed can be obtained.
FIG. 15 shows the layout of the pixel region 601 of the image pickup device. As shown in FIG. 15, the pixel region 601 includes an effective pixel area 801 comprised of pixels for converting an image formed thereon by a photographing lens, not shown, into electrical signals. A VOB 802 is formed by light-shielded pixels and disposed adjacent to the effective pixel area 801 in the vertical direction. The VOB 802 is for detecting a black level and correcting an offset variation in the output of the image pickup device due to a variation in dark current component or a temperature variation.
An HOB 803 is formed by light-shielded pixels as in the case of the VOB 802, is disposed adjacent to the effective pixel area 801 in the horizontal direction, and is used for correcting a vertical dark shading component (see, for example, Japanese Laid-open Patent Publication No. 2000-152098).
A dark shading component can appear due to a dark current shading, and especially in a CMOS image pickup device, it appears due to a voltage shading caused by an impedance of a power supply line.
To detect offset values for respective lines from pixel data in the HOB region, it is necessary to use a sufficient number of pixel data large enough to eliminate affections by random noise and pixel defects. Since the offset values must be detected on a line-by-line basis, pixel data in preceding and subsequent lines cannot be used, and therefore an adequate HOB region must be ensured for each line. Thus, an extremely large number of pixels are required as a whole, which increases the chip area of the image pickup device and results in increase in the cost thereof.